The occupation fraction of massive black holes (MBHs) in low mass galaxies
offers interesting insights into initial black hole seeding mechanisms and
their mass assembly history, though disentangling these two effects remains
challenging. Using the Romulus cosmological simulations we examine the impact
of environment on the occupation fraction of MBHs in low mass galaxies. Unlike
most modern cosmological simulations, Romulus seeds MBHs based on local gas
properties, selecting very dense, pristine, and rapidly collapsing regions in
the early Universe as sites to host MBHs without assuming anything about MBH
occupation as a function of galaxy stellar mass, or halo mass, a priori. The
simulations predict that dwarf galaxies with Mโโ<109 Mโโ in
cluster environments are approximately two times more likely to host a MBH
compared to those in the field. The predicted occupation fractions are
remarkably consistent with those of nuclear star clusters. Across cluster and
field environments, dwarf galaxies with earlier formation times are more likely
to host a MBH. Thus, while the MBH occupation function is similar between
cluster and field environments at high redshift (z>3), a difference arises as
late-forming dwarfs -- which do not exist in the cluster environment -- begin
to dominate in the field and pull the MBH occupation fraction down for low mass
galaxies. Additionally, prior to in-fall some cluster dwarfs are similar to
progenitors of massive, isolated galaxies, indicating that they might have
grown to higher masses had they not been impeded by the cluster environment.
While the population of MBHs in dwarf galaxies is already widely understood to
be important for understanding MBH formation, this work demonstrates that
environmental dependence is important to consider as future observations search
for low mass black holes in dwarf galaxies.Comment: 16 pages, 7 figures, to be submitted to the Open Journal of
Astrophysic